Rescuers in Texas were set to return to the rubble today in the search for survivors after a massive blast at a fertilizer factory killed as many as 15 people and destroyed dozens of homes.

Rescuers in Texas were set to return to the rubble on Friday in the search for survivors after a massive blast at a fertilizer factory killed as many as 15 people and destroyed dozens of homes.

With the country already on edge after the deadly Boston Marathon attacks, the factory exploded on Wednesday with the force of a 2.1 magnitude earthquake, devastating much of the small town of West and sending up a toxic cloud.

The town was in mourning, gathering with friends and neighbors at vigils Thursday night, even as officials refused to release a death toll and rumors flew about who was missing and which bodies have been found.

"It's very hard not knowing who's alive and who isn't," said Julie Veselka, 34, hugging her infant daughter while tears streamed down her face, after a packed service at St. Mary's church concluded.

Authorities said they feared they could find more bodies in the rubble of homes and businesses leveled by the explosion, which may have been sparked by a fire that broke out at the West Fertilizer plant in the southwest US state.

Much of West was evacuated overnight as an acrid cloud hung over the area, and Texas Governor Rick Perry said local schools would remain closed for the remainder of the week.

"Last night was truly a nightmare scenario for that community," Perry told a news conference in the state capital Austin, announcing that he was seeking a federal disaster declaration that would make additional funds available.

"This tragedy has most likely hit every family, it has touched practically everyone in that town," Perry said.

Police Sergeant W. Patrick Swanton of nearby Waco said the tragedy killed "anywhere from five to 15" people, but said he expected that toll to rise. Hospitals have treated more than 160 casualties with varying injuries, he said.

McLennan County Deputy Sheriff Matt Cawthon told reporters the devastated area had been "highly populated" and was "still a very volatile situation" because of the presence of ammonium nitrate, a common but potentially explosive fertilizer ingredient.

A US National Guard contingent of 20 troops trained to aid in emergencies and incidents involving weapons of mass destruction was dispatched to the scene to monitor for hazardous emissions.

Officials said they do not yet know what caused the explosion, but are treating the factory site as a crime scene until they rule out foul play.

The West Fertilizer Company paid more than $5,000 in fines in 2012 after being cited for mislabeled cargo tanks and inadequate transport practices, and had been cited by state authorities for a lack of permit in 2006.

The factory reportedly held large quantities of anhydrous ammonia, a pungent, colorless gas stored in pressurized tanks that can ignite in dense concentrations and under high heat.

The blast was felt up to 50 miles (80 kilometers) away, and an expert at the US Geological Survey told AFP the force of the explosion had registered as a 2.1 magnitude seismic event.

Some 60-75 people have been left homeless by the disaster, according to Mark Felton, director of the local Red Cross chapter.

Pete Arias, 45, was grateful to still be alive after the explosion blew out the windows of his modest one-story house. Somehow, he and his eight-year-old son escaped injury from what they initially thought was an earthquake.

"The force that came in stripped the paint off the ceiling," he told AFP.

President Barack Obama offered the prayers of the nation to the town of about 2,800 people, saying "a tight-knit community has been shaken, and good, hard-working people have lost their lives."

The explosion at West Fertilizer came with the nation still raw with emotion after the Boston Marathon bombings Monday, which left three people dead and scores maimed.

Americans also were on edge after letters apparently laced with the deadly poison ricin were sent to Obama and a US senator. A 45-year-old Mississippi man has been arrested and charged with threatening the president's life.

The area around the blast was closed off by police, but a Texas public security spokesman D. L. Wilson, one of the first people on the scene, described the devastation as "massive."

"Just like Iraq. Just like the Murrah building in Oklahoma City," he said.

The April 19, 1995, truck bombing of the Alfred P. Murrah federal building in Oklahoma City by anti-government extremists killed 168 people, including 19 children, and injured hundreds more.

Friday is also the 20th anniversary of a deadly confrontation in Waco between federal authorities and heavily armed members of the Branch Davidian religious group, a botched showdown that inspired the Oklahoma City bombing.